Method for joining thermoplastic sheet material



1954 s. s. MINER ETAL 2,698,273

METHOD FOR JOINING THERMOPLASTIC SHEET MATERIAL Filed Oct. 19, 1949 5Sheets-Sheet 1 INVENTORS ATTORNEY 1360- ,1954 s. s. MINER ETAL METHODFOR JOINING THERMOPLASTIC SHEET MATERIAL Filed 001;. 19, 1949 3Sheets-Sheet 2 van a 'IIII Dec. 28, 1954 s. s. MINER ErAL METHOD FORJOINING THERMOPLASTIC SHEET MATERIAL Filed 001;. 19, 1949 3 Sheets-Sheet3 ATTORNEY United States Patent METHGD FOR JUINING THERMOPLASTIC SHEETMATERIAL Samuel S. Miner and Edward H. Clark, Mishawaka, Ind., assignorsto .United States Rubber Company, New York, N. Y., a corporation of NewJersey Application Uctohar 19, 1e49, Serial No. 122,212

8 claims. (or. 154-116 This invention relates to a method and apparatusfor joining thermoplastic sheet material, particularly sheetpolyethylene, especially of substantial thickness, i. e., of a thicknessgreater than that of foil, typically ranging fIiOgl 0.01 inch upwardlyto any desired thickness, say 0.5 we

The apparatus of this invention is described and claimed in .copendingdivisional application Serial No. 426,617, filed April 22, 1954.

The problem of making satisfactory joints between sheets of polyethylenehas existed for a considerable period of time. Polyethylene has a verysharp melting point. Just below the melting point it is a solid, whilejust above it, it is a free-flowing liquid. Therefore, the usual type ofheat-sealing devices, which are satisfactory for joining suchthermoplastic materials as polyvinyl chloride or copolymers of vinylchloride and vinyl acetate, which only gradually become free-flowingliquids as the temperature is raised, will not work on polyethylene.

U. S. Patent 2,520,737 issued August 29, 1950 to Romeyn et al. andassigned to the assignee of the present invention, describes what isbelieved to be the first successful method of joining sheets ofpolyethylene of substantial thickness. In the method of that patent, thejoining is effected by a very careful control of temperature andpressure, specifically by heating the material at the interface of theoverlap to a temperature of not less than C. below and not more than C.above the melting point While applying pressure at the overlap, theheating and application of pressure being continued for a period of timesubstantially only that required to produce a satisfactory joint.

The present invention provides a method and apparatus for the joining ofpolyethylene sheets of substantial thickness without the precise controlof temperature and pressure required by the above-mentioned patent.

While the invention is especially adapted to joining polyethylenesheets, the joining of which presents a particularly diflicult problem,it may if desired be employed to effect homogeneous and integral joiningof sheets of other thermoplastic materials which do not present thedifficulties attendant upon the successful joining of polyethylenesheets. Examples of such other thermoplastic materials which may bejoined by following the principles of the present invention are:polyvinyl chloride, copolymers of vinyl chloride and vinyl acetate,polyvinyl acetals, e. g., polyvinyl butyral, polyvinylidene chloride,polystyrene, acrylic resins such as polymeric methyl acrylate, polymericmethyl methacrylate and similar polymeric esters of acrylic acid andmethacrylic acid, cellulose acetate, ethyl cellulose, etc.

In splicing sheets of thermoplastic material, the seam area Where thesheets are lapped to form the joint is twice as thick as the singlesheet of thermoplastic material, and, therefore, twice as much heat mustbe supplied as would be required to soften a single layer of thematerial. Consequently, the areas adjacent to the splice become very hotand soften much in advance of the lapped splice itself. For this reasonsplices in thermo-. plastics, especially those having a sharp meltingpoint and low thermal conductivity like polyethylene, are very diiiicultto make. The present invention, however, overcomes this difficulty andmakes it possible to effect such splices in a simple and commerciallyfeasible manner.

Th Pre en n en i n s Par cu arl adv ntageo s because it enables thejoining of polyethylene sheet maice terials so as to form an integralhomogeneous one piece container of any desired size. Such containers areof extreme value for the storage and transportation of corrosivechemicals and for this reason the present invention is especiallyimportant since it provides a method of making such containers with easeand facility.

In the drawing:

Fig. 1 is a top plan view of a device, constructed in accordance withthe invention, for making a lap joint between two flat sheets ofthermoplastic material.

Fig. 2 is a longitudinal section on the line 22 of Fig. l; theappearance of the platen-carrying portion of the device when open isportrayed in dotted lines.

Fig. 3 is a transverse section on the line 33 of Fig. 1.

Fig. 4 is a side view, with the joint-forming portion in longitudinalsection, of a unit for making endless circular joints; this unit isespecially adapted for joining the domed ends of a chemically resistantthermoplastic tank to a cylindrical body portion; it can also be used tojoin two cylindrical body portions to form a longer cylindrical body.

Fig. 5 is a front view of the joint-forming portion of the unit of Fig.4, taken on the line 55 of Fig. 4.

Fig. 6 is a section on the line 6--6 of Fig. 5.

Fig. 7 is a vertical sectional view of a unit for joining a flaliligedcollar to a flange projecting from the body of a ce The presentinvention is based upon the discovery that thermoplastic sheetmaterials, especially polyethylene in sheet form at least 0.01 inchthick, may be integrally and homogeneously joined to form a joint whichis just as strong as the remainder of the assembly, by lapping twosurfaces of the sheet material, heating the material in the area of thejoint to a temperature substantially above its softening point andthereby causing it to fuse and flow together at the site of the joint,simultaneously maintaining the areas of the material immediatelyadjacent to and on each side of the area of the joint at a temperaturesubstantially below the softening point by cooling and confining thematerial at the joint under uniform pressure as it is being melted,while it is in molten condition, and thereafter during the subsequentcooling step until the material has cooled to a temperaturesubstantially below its softening point. After the joint has beeneffected in this manner, heating is discontinued and the joint is cooledto a temperature substantially below its melting point in order tosolidify the material forming the joint (which was melted and caused toflow together under the, heat and, pressure) and permit release ofpressure and removal of the joint from the equipment.

In the preferred practice of the invention, uniform fluid pressure isapplied to the material in the area of the joint and to the cooledadjacent areas by means of a flexible inflatable pressure-resisting bagwhich is in-- fiated by a gas under pressure, usually air.

The invention is based upon the discovery that when the thermoplasticsheet material at the joint is heated up to above the softening point sothat it becomes liquid, the normal tendency of the thermoplasticmaterial to flow outunder the pressure exerted at the joint may beprevented in a simple and economical way by cool ing at each side of thejoint so that the hot flowing thermoplastic material is. cooled quickly,causing it to harden up at the edges of the joint. If these edges arenot cooled in this way the molten material, at the joint would flow out,under the pressure exerted, to such an extent that there would bepractically no material left at the site of the joint. In the inventiondescribedin the patent of Romeyn et al., mentioned above, it wasnecessary to apply both heat and pressure judiciously. With the presentinvention the problem of holding the temperature and pressure withincritical limits is eliminated, since in the present invention it doesnot matter how fluid the material in the area of the joint becomes orhow much pressure is exerted upon this molten material because theprovision of cooling on each side; of the joint and the exertion ofpressure across the lines between the joint and the cooled areasprevents the escape of the molten. thermoplastic.

The, cooled material along the edges. of the joint serves completely toconfine the molten material between the boundaries of the heatingplaten, the pressure bag and the lateral cooled portions. Thus there areno mechanical confining means along the lateral edges of the joint, thematerial itself serving as the lateral barriers.

The drawings portray three different embodiments of apparatus forcarrying out the present invention. These different embodiments arespecially designed for effecting particular types of joints.

Referring now to the embodiment portrayed in Figs. 1 to 3 in thedrawings, there is provided a lower stationary platen 1 fixedlysupported in any suitable man ner on support 2 and an upper platenmember 3 which is pivoted at one end at 4 to one end of lower platen 1.Platen-carrying member 3 fixedly carries on its lower face a heatingplaten 5, heated by electrical resistance element 6, and two coolingplatens 7 and 8 on each side of heating platen 5. Cooling platens 7 and8 are cooled by the circulation of a coolant liquid through the pipes 9,the coolant entering and leaving via flexible tubes 10 and 11. Theheating platen 5 and the cooling platens 7 and 8 are insulated from eachother by insulation 12 and from the supporting member 3 by insulation13.

The lower platen 1 is hollowed out to receive a rubber bag 14 equippedwith valve 15 and supplied with inflaring gas (almost invariably air)under pressure by flexible tube 16 for the purpose of applying pressureacross the area of the joint and the adjacent cooled areas.

The upper platen member 3 is provided at its free end with a hook 17which is adapted to lock the upper and lower members 3 and 1 together toresist the pressure exerted by the bag 14 while a joint is being formed.

In operation, two pieces of thermoplastic sheet material, 18 and 19 arelaid across bag 14 while the press is open. The press is then closed,bag 14 is inflated, and platen 5 is heated to above the softening pointof the thermoplastic material while platens 7 and 8 are cooled by thecirculation of coolant through pipes 9 to hold the material at the sidesof platen 5 at a temperature well below the softening point. After theseam has been made, the heating of platen 5 is discontinued until thejoint has been cooled to a temperature well below the softening pointafter which the pressure is released and the press opened.

In the embodiment portrayed in Figs. 4 to 6 a circular seam is shown asbeing made between a cylindrical body portion 20 and a domed end cap 21having a fitting opening 22, by a unit which utilizes the principlesdescribed above but which is especially adapted to the formation ofcircular joints. The unit comprises an annular ring or wheel 23comprising three segments held together by pins 24. By removal of pins24, the ring 23 can be collapsed inside the formed container, the threesegments of ring 23 being removable through the relatively small fittingopening 22. An oblique break 23a is provided between two of the threesegments of ring' 23 in order to permit assembly and dis-assembly of thewheel 23.

Ring 23 is held in position by the three radial rods 25 which arepivoted at their inner ends to the enlarged end of a sleeve 26 so thatrods 25 are moved from the radial position to the horizontal positionwhen sleeve 26 is moved rightwardly of Fig. 4 along stationary shaft 27.The position of the rods 25 when they are thus unlocked from supportingengagement with wheel 23 is shown in dotted lines in Fig. 4. It will beunderstood that the outer ends of rods 25 are rounded slightly andengage slightly rounded depressions in lugs 28 which. are integrallyattached to the inside faces of the sections of the wheel 23. Thus therods 25 can be moved into and out of supporting engagement with wheel 23by the longitudinal movement of sleeve 26 which may be effected by acrank 29 actuated by a rod 30 which may be connected to a foot treadle(not shown). A safety latch 31 may be provided to prevent accidentalmovement of sleeve 26.

The ring 23 has on its outer face an annular channel in which isdisposed the annular inflatable rubber pressure bag 32 supplied with airunder pressure by tube 33 and the usual valve.

Surrounding ring 23 is an external ring member 34 disposed exteriorly ofthe portions 20 and 21 being joined. Ring 34 carries the heating platen35 and the cooling platens 36 on each side thereof and separatedtherefrom by insulation 37.

Ring 34 is supported by an eye member 38 through which passes horizontalsupport 39. Support 39 can be adjusted by means of turnbuckle 40, so asto center ring 34 with respect to wheel 23. Notch 39a is formed in theupper face of support 39 to receive eye 38 and prevent undesiredlongitudinal movement of ring 34 out of alignment with Wheel 23.

Platens 36 are cooled by coolant liquid supplied and withdrawn byflexible tubes 41 and 42, and circulated through annular cooling tubes43 in platens 36. Platen 35 is heated by electrical resistance element44.

Arm 39 and shaft 27 are supported by vertical support 45.

The operation of the unit of Figs. 4 to 6, when making a cylindricalthermoplastic vessel having domed ends, is as follows. The equipment andsections 20 and 21 are first arranged as shown in Fig. 4, the end cap 21having opening 22 being applied before the other end cap (not shown butwhich has no opening), to permit assembly. Bag 32 is inflated andplatens 35 and 36 are heated and cooled, respectively, to effect thejoining. Application of heat to platen 35 is then discontinued and thejoint is then cooled down to well below the softening point. Bag 32 isthen deflated and spokes 25 are retracted to the dotted position of Fig.4 by releasing latch 31 and pulling down on rod 30. Then the assemblycomprising joined sections 20 and 21 and the wheel 23 is removed andreversed from the position shown in Fig. 4, the opening 22 nowsurrounding sleeve 26. Wheel 23 is now moved into longitudinal alignmentwith platencarrying ring 34 and with the overlapping portions of section20 and the other end section (not shown, but which is now in theposition formerly occupied by section 21) which are to be joined. Spokes25 are then moved into radial supporting engagement with Wheel 23. Theair bag 32 is then inflated to force the material at and adjacent thejoint against the platens 35 and 36. The joint is next effected. Afterthe joint has been made and cooled down to below the softening point,the spokes 25 are retracted again to the dottedline position, bag 32 isdeflated, and the assembly including the completed container with wheel23 inside it is removed from the machine and taken to the bench wherepins 24 are withdrawn and the three sections of th; collapsible wheel 23are removed through opening 2 It is often preferred to lay a sheet ofnon-thermoplastic foil, preferably one of aluminum because of itsexcellent heat-transmitting characteristics, across the sectionalplatens of the units portrayed in Figs. 1 to 6, in order to prevent theinsulation 12 and 37 between the sections from leaving an impression inthe splice.

Referring now to the embodiment of Fig. 7, this is employed to weldflange 50 of an angular collar 51 to flange 52 projecting outwardly froma depressed portion 53 of a body 54. Flange 50 and flange 51 areoverlapped and placed in the press as shown. The annular rubberinflation bag 55 is square in cross-section and when inflated appliespressure not only to the overlapped area but to the horizontal portionsof the collar 51 and of the depressed portion 53 as well. Inflation airis supplied to bag 55 by tube 55a which enters through ring 60. Theannular heating platen 56, heated by element 57, is adjacent to theoverlap in vertical position and the two cooling platens 58 and 59 arepositioned at right angles to platen 56 so as to cool any material whichWould tend to flow out from the splice area. Ring 60, surrounding bag 55and upper cooling platen 58, restrains the bag and enables the bag wheninflated to exert pressure on the joint and on the adjacent cooledareas. Ring 60 also functions as a spacer to limit the application ofmechanical pressure to the press. Ring 60 is so arranged as to admit thematerial 53 between it and the cooling platen 59. Cooling platen 59 isfixedly carried on a bottom support 61 While cooling platen 58 isfixedly carried on an upper pressure-applying member 62. Pressure isapplied to element 62 by hand wheel 63 which screW-threadedly engagescentral post 64 which is fixedly attached as by welding 65 to bottommember 61.

The operation of the obvious from the foregoing but may be brieflysummarized as follows:

embodiment of Fig. 7 will be The'member 61; isfirst. positioned insidethe body 54,.

hich should be open at one, end, with the. portion 53 along; cooling.platen 5.9 as shown. Then ring 60, carry-a ing bag; 55,, and. ringdike:heating platen 56 are placed into the position shown. Then the flangedcollar 51 is inserted. after which the pressure-applying element 62 is.placed; into position over post 64. Pressure is then applied by means ofhandwheel 63. Then bag 55 is inflated and the cooling platens andheating platen 56 is heatedv to effect the joint. Thereafter operationis as before, element 61. being finally removed from within the body 54,through an open end thereof. It will be obvious that one or both of thecooling platens 58 and 59 may be, disposed at an. angle other than aright angle to the heating, platen 56, when it is desired to form a.shape other than that portrayed in- Fig. 7.

It will also be obvious that if desired the cooling platens 7 and, 8.(Figs. 1 to 3) and 58 and 59 (Fig, 7) may be grooved on the side next tothe cooling tubes in a. manner similar to that shown in Fig. 6 in orderto assure intimate contact and good heat conductivity between thecooling tubes and the cooling platens.

Typical technique with polyethylene In a. typical example of making asplice from sheets of commercial polyethylene, 0.1 in. in thickness,pressure of about 5. lbs/sq. in. is first applied to the air bag. Thenthe heating element is turned on. The air pressure. is then increased toabout lbs/sq. in. as the splice heatsup. This serves to squeeze outtrapped air. Cooling Water is introduced into the side sections of theunit shortly after the center section has started to heat. About 5minutes elapse from the time the heating element' is' turned on untilthe polyethylene at the splice is completely fused and flowed together.Then the power in the heating element is cut off and the rate at whichwater flows through the cooling elements is increased in order to cooldown the entire splice as rapidly as possible. This requires about 6minutes. The temperature of the splice during fusion is between 250 and350 F. The area beneath the cooled sections of the platen is maintainedat about 150 to 180 F. while the splice is being effected. After thesplice has been completed it is cooled to below approximatelyt 100 F.before it is removed.

General The cycle given for commercial polyethylene in the precedingparagraph will varywhere other thermoplastics are used. For instance,when sheets of a mixture of polyisobutylene (e. g., Vistanex) andpolyethylene are to be spliced, a higher temperature must be used sincesuch a mixture softens at a higher temperature than polyethylene. Thetime and temperature will also vary with the type of splice. Forinstance, the circular splice takes a longer time than a straight linesplice because there is more mass to heat up in the ring platejn. Ofcourse, all these variations can be made by the operator according tothe particular needs. Those skilled in the art will be enabled to effectany desired splice by following the teachings of this specification.

In general, the heating platen is heated to a temperature substantiallyabove the softening point, i. e., sufficiently thereabove to make thematerial completely molten so that it flows together to form a perfectlyunbroken joint. It is preferred to heat the material at the joint to atleast 10 F. above the softening or melting point, typically to from 10to 110 F. thereabove.

In general, the cooling platens are cooled to such an extent as tomaintain the material at the sides of the splice at a temperaturesubstantially below the softening point, i. e., sufliciently therebelowthat the material cannot flow under the pressure exerted by the bag. Itis preferred to cool to such an extent as to keep the temperature of thematerial at the sides of the joint at least as low as 50 F. below thesoftening or melting point, commonly to from 50 to 100 F. below themelting point.

In general, the joint is finally cooled to at least 50 F. below themelting point and often to at least 130 F. therebelow before releasingthe pressure.

As indicated above, in the case of commercial polyethylene, having amelting point of 115 C. (239 F.), it is often preferred to heat thejoint to 250 to 350 58 and 59 are cooled.

6). F.,. to cool the. adjacent. areas to 150: to 180?, and to finally:cool the. joint. to below F. before: removing it.

In, general sufficient to cause the. molten thermoplastic to flowtogether togive av unitary joint and to. restrain and con fine themolten material at the splice until it hassolidified tov such. an extentas: to be. incapable of flowing-under the pressure applied. Usually thepressure applied during splicing will be within the range of from 10 to100 lbs/squirt.

By' the present invention heat, cooling and pressure are applied so asto cooperate in a new way to produce results never before achieved. Thesimultaneousmelting at thev joint, cooling of adjacent areas andapplication of' uniform fluid pressure across the joint by means of theinflated rubber bag so cooperate that a perfectly homo-' geneous jointis produced in a simple and commercially feasible manner.

Having thus described our invention, what we claim and desire to protectby Letters Patent is:

1'. A process of joining thermoplastic sheet material having a sharpmelting point which comprises lappingtwo surfaces. ofsaid material,heating the material in the area of the joint to a temperaturesubstantially above its softening point and thereby causing it to fuseand flow together to form anintegral joint, simultaneously maintainingthe material. immediately adjacent tothe area of the joint on both.sides thereof: at a temperature substantially below its softening pointby cooling, applying substantially uniform fluid pressure to thematerial at the joint before it is molten, confining the material at thejoint and along the cooled. adjacent areas under uniform fluid pressurewhile the material at the joint is molten and thereafter untilit. hascooled to atemperature substantially below its softening point, andsubsequently discontinuing said heating and cooling the resulting jointto a temperature substantially below its softening point.

2. A process of joining polyethylene sheet material which compriseslapping two surfaces of said material, heating the material in the areaof the joint to a temperature substantially above its softening pointand thereby causing it to fuse and flow together to form an integraljoint, simultaneously maintaining the material immediately adjacent tothe area of the joint on both sides thereof at a temperaturesubstantially below its softening point by cooling, applyingsubstantially uniform fluid pressure to the material at the joint beforeit is molten, confining the material at the joint and along the cooledadjacent areas under uniform fluid pressure while the material at thejoint is molten and thereafter until it has cooled to a temperaturesubstantially below its softening point, and subsequently discontinuingsaid heating and cooling the resulting joint to a temperaturesubstantially below its softening point.

3. A process of joining material consisting of polyethylene in sheetform at least 0.01 inch thick which comprises lapping two surfaces ofsaid polyethylene, heating the polyethylene in the area of the joint toa temperature substantially above its softening point and therebycausing it to fuse and flow together to form an integral joint,simultaneously maintaining the polyethylene immediately adjacent to thearea of the joint on both sides thereof at a temperature substantiallybelow its softening point by cooling, applying substantially uniformfluid pressure to the material at the joint before it is molten,confining the polyethylene at the joint and along the cooled adjacentareas under uniform fluid pressure while the polyethylene at the jointsis molten and thereafter until it has cooled to a temperaturesubstantially below its softening point, and subsequently discontinuingsaid heating and cooling the resulting joint to a temperaturesubstantially below its softening point.

4. A process as set forth in claim 3 wherein the polyethylene at thejoint is heated to a temperature of from 250 to 350 F. and the areasadjacent the joint are maintained at from to 180 F. during fusion of thepolyethylene at the joint, and wherein the joint is cooled to below 100F. before the pressure is released.

5. A process of joining thermoplastic sheet material having a sharpmelting point which comprises lapping two surfaces of said material,heating the material in the area of the joint to a temperaturesubstantially above its 85 softening point and thereby causing it tofuse and flow the pressure applied by the bag should be together to forman integral joint, simultaneously cooling the material in the areasimmediately adjacent to the area of the joint on both sides thereof tomaintain a temperature substantially below the softening point of saidmaterial, applying uniform fluid pressure to the material in bothvtheheated and cooled areas during said heating and cooling steps and duringthe subsequent cooling step until the thermoplastic material at thejoint has cooled to a temperature substantially below its softeningpoint, after the material at the joint has flowed together to ahomogeneous mass cooling the resulting joint to a temperaturesubstantially below the softening point of said material, and thereafterreleasing the pressure.

6. A process of joining polyethylene sheet material which compriseslapping two surfaces of the material to be joined, heating the materialin the area of the joint to a temperature substantially above thesoftening point of the polyethylene material and thereby causing it tofuse and flow together to form an integral joint, simultaneously coolingthe material in the areas immediately adjacent to the area of the jointon both sides thereof to maintain a temperature substantially below thesoftening point of said material, applying uniform fluid pressure to thematerial in both the heated and cooled areas during said heating andcooling and during the subsequent cooling step until said material atthe joint has been cooled to a temperature substantially below itssoftening point, after the material at the joint has flowed together toa homogeneous mass cooling the resulting joint to a temperaturesubstantially below the softening point of said material, and thereafterreleasing the pressure.

7. A process of joining material consisting of polyethylene in sheetform at least 0.01 inch thick which comprises lapping two surfaces ofthe material to be joined, heating the material in the area of the jointto a temperature substantially above the softening point of thepolyethylene and thereby causing it to fuse and flow together to form anintegral joint, simultaneously cooling the polyethylene in the areasimmediately adjacent to the area of the joint on both sides thereof tomaintain a temperature substantially below the softening point of saidpolyethylene, applying uniform fluid pressure to the material in boththe heated and cooled areas during said heating and cooling and duringthe subsequent cooling step until the polyethylene at the joint hascooled to a temperature substantially below its softening point, afterthe polyethylene at the joint has flowed together to a homogeneous masscooling the resulting joint to a temperature substantially below thesoftening point of said polyethylene, and thereafter releasing thepressure.

8. A process as set forth in claim 7 wherein the polyethylene at thejoint is heated to a temperature of from 250 to 350 F. and the areasadjacent the joint are maintained at from to 180 F. during fusion of thepolyethylene at the joint, and wherein the resulting joint is cooled tobelow 100 F. before releasing said pressure.

References Cited in the file of this patent UNITED STATES PATENTS

1. A PROCESS OF JOINING THERMOPLASTIC SHEET MATERIAL HAVING A SHARPMELTING POINT WHICH COMPRISES LAPPING TWO SURFACES OF SAID MATERIAL,HEATING THE MATERIAL IN THE AREA OF THE JOINT TO A TEMPERATURESUBSTANTIALLY ABOVE ITS SOFTENING POINT AND THEREBY CAUSING IT TO FUSEAND FLOW TOGETHER TO FROM AN INTEGRAL JOINT, SIMULTANEOUSLY MAINTAININGTHE MATERIAL IMMEDIATELY ADJACENT TO THE AREA OF THE JOINT ON BOTH SIDESTHEREOF AT A TEMPERATURE SUBSTANTIALLY BELOW ITS SOFTENING POINT BYCOOLING, APPLYING SUBSTANTIALLY UNIFORM FLUID PRESSURE TO THE MATERIALAT THE JOINT BEFORE IT IS MOLTEN, CONFINING THE MATERIAL AT THE JOINTAND ALONG THE COOLED ADJACENT AREAS UNDER UNIFORM FLUID PRESSURE WHILETHE MATERIAL AT THE JOINT IS MOLTEN AND THEREAFTER UNIT IT HAS COOLED TOA TEMPERATURE SUBSTANTIALLY BELOW ITS SOFTENING POINT, AND SUBSEQUENTLYDISCONTINUING SAID HEATING SAID COOLING THE RESULTING JOINT TO ATEMPERATURE SUBSTANTIALLY BELOW ITS SOFTENING POINT.